If you are using Microsoft Windows O.S then you might have seen “Blue Screen of Death” ( BSOD). I have experienced it 3 times in my own system and have seen many times in my friends system too. If you have not seen this yet, it means you are lucky that your system is working fine :P.  But it’s good to know that what are the causes that leads us towards BSOD? It may happen for various causes, few are listed here:

1.  The number one reason why Windows crashes is hardware conflict. Each hardware device communicates to other devices through an interrupt request channel (IRQ). These are supposed to be unique for each device.

For example, a printer usually connects internally on IRQ 7. The keyboard usually uses IRQ 1 and the floppy disk drive IRQ 6. Each device will try to hog a single IRQ for itself. If there are a lot of devices, or if they are not installed properly, two of them may end up sharing the same IRQ number. When the user tries to use both devices at the same time, a crash can happen.

2. Bad Ram Ram (random-access memory) problems might bring on the blue screen of death with a message saying Fatal Exception Error. A fatal error indicates a serious hardware problem. Sometimes it may mean a part is damaged and will need replacing.

3. BIOS settings Every motherboard is supplied with a range of chipset settings that are decided in the factory. A common way to access these settings is to press the F2 or delete button during the first few seconds of a boot-up. Once inside the BIOS, great care should be taken. It is a good idea to write down on a piece of paper all the settings that appear on the screen.
That way, if you change something and the computer becomes more unstable, you will know what settings to revert to. MS  Windows is better at allocating IRQ numbers than any BIOS. If possible set the IRQ numbers to Auto in the BIOS. This will allow Windows to allocate the IRQ numbers (make sure the BIOS setting for Plug and Play OS is switched to ‘yes’ to allow Windows to do this.).

4. Hard disk drives After a few weeks, the information on a hard disk drive starts to become piecemeal or fragmented. It is a good idea to defragment the hard disk every week or so, to prevent the disk from causing a screen freeze. There are many tools to defragment it a.       Disk Defragmenter b.      ScanDisk

5. Viruses Often the first sign of a virus infection is instability. Some viruses erase the boot sector of a hard drive, making it impossible to start. This is why it is a good idea to create a Windows start-up disk. Use any good anti-virus for it. I use Eset NOD 32 . McAfee, Norton are also very good in that.

6. Software A common cause of computer crash is faulty or badly-installed software. Often the problem can be cured by uninstalling the software and then reinstalling it. The System Registry can be corrupted by old references to obsolete software that you thought was uninstalled. Ccleaner  to clean up the System Registry and remove obsolete entries. It works on Windows 95, Windows 98, Windows 98 SE (Second Edition), Windows Millennium Edition (ME), NT4 and Windows 2000.

7. Overheating Central processing units (CPUs) are usually equipped with fans to keep them cool. If the fan fails or if the CPU gets old it may start to overheat and generate a particular kind of error called a kernel error. This is a common problem in chips that have been over clocked to operate at higher speeds than they are supposed to. One remedy is to get a bigger better fan and install it on top of the CPU. Specialist cooling fans/heatsinks are available from www.computernerd.com or www.coolit.com CPU problems can often be fixed by disabling the CPU internal cache in the BIOS. This will make the machine run more slowly, but it should also be more stable.

8. Power supply problems With all the new construction going on around the country the steady supply of electricity has become disrupted. A power surge or spike can crash a computer as easily as a power cut. If this has become a nuisance for you then consider buying a uninterrupted power supply (UPS). This will give you a clean power supply when there is electricity, and it will give you a few minutes to perform a controlled shutdown in case of a power cut. It is a good investment if your data are critical, because a power cut will cause any unsaved data to be lost.

Source : http://faq.programmerworld.net/pc/10-reasons-why-pcs-crash.html